The present invention relates to a radiation dose monitor to measure distribution of radiation dosage.
FIG. 1 shows a conventional radiation dose monitor as disclosed in U.S. Pat. No. 3,808,441. In the ionization box housing (not shown), a power supply electrode 1 is provided to apply an electric field to the ions produced by radiation, a first pair of semi-circular collector electrodes 3 are separated by a band-shaped gap 3a and positioned above the power supply electrode 1 across a first ionization chamber 2, and a second pair of semi-circular collector electrodes 5 are separated by a band-shaped gap 5a and are arranged below the power supply electrode 1 across a second ionization chamber 4. The direction in which the second collector electrode 5 is divided at 5a is perpendicular to the direction in which the first collector electrode 3 is divided at 3a.
In the radiation dose monitor of the above construction, the gas in the first ionization chamber 2 and second ionization chamber 4 is ionized by a radiation beam passing vertically with respect to the power supply electrode 1. When a high positive DC voltage is applied to the power supply electrode 1, negative ions in the first and second ionization chambers 2 and 4 move to the electrode 1. At the same time, positive ions in the first ionization chamber 2 move to the first collector electrodes 3, while positive ions in the second ionization chamber 4 move to the second collector electrodes 5. Thus, ion currents flow. These ion currents are then transferred from the first collector electrode 3 and second collector electrode 5 to a measurement circuit, not shown, where the radiation dose can thus be measured from the values of the currents flowing through the collector electrodes 3 and 5, respectively. More specifically, the first and second collector electrodes 3 and 5 measure the radiation dose distribution of the radiation in first and second directions perpendicular to each other.
FIG. 2 shows another example of a conventional radiation monitor which can measure both the radiation dose distribution and total radiation dose, and it has a total radiation measurement power supply electrode 7 arranged below the second collector electrode 5 across a third ionization chamber 6. A total radiation collector electrode 9 of non-divided construction is arranged below the total radiation power supply electrode 7 across a fourth ionization chamber 8.
When a high positive DC voltage is applied to the total radiation power supply electrode 7 as well as to the power supply electrode 1, positive ions in the fourth ionization chamber 8 move to the total radiation collector electrode 9, and the resulting ion current is transferred from the electrode 9 to a measurement circuit, not shown. The radiation dose distribution is then found from the values of the ion currents flowing to the first collector electrodes 3 and the second collector electrodes 5, while the total radiation can be found from the value of the ion current flowing to the total radiation collector electrode 9.
Conventional radiation dose monitors of the construction described above require, in the first example, first and second ionization chambers 2 and 4, and in the second example, first, second, third and fourth ionization chambers 2, 4, 6 and 8, with the result that the whole device takes up a great deal of space.